1. Field of the Invention
This invention relates to rotating electrical machines.
More particularly the invention relates to rotating electrical machines rated at powers in excess of 10 kilowatts, of the kind having a rotor comprising conductor bars of copper or similar conducting material located in slots in a rotor core of magnetic material and connected together to form a winding. These will be referred to below as electrical machines of the kind specified. Examples of such machines are induction motors wherein the rotor conductor bars are connected at their ends to form a cage winding and synchronous motors wherein the rotor conductors are connected to form a damper cage winding.
2. Description of the Related Art
During manufacture of the rotor, it is customary to insert the copper or aluminum conductor bars by force-fitting them into the slots, thereby slightly distorting the bars and/or the slots. This procedure is intended to ensure a very tight fit of the bars into the slots, thereby obviating any relative movement between them and so enabling the bars to remain fixed securely in the slots throughout the service life of the machine.
In such machines the rotor conductor bars are usually uninsulated. However, in recent years it has become apparent that in some circumstances sparking can occur between the uninsulated conductor bars and the rotor core. Surprisingly, such sparking can occur even when uninsulated bars have previously been tightly located in their slots in the rotor core. Such sparking is unacceptable in machines required to operate in explosive atmospheres.
It has been discovered that such sparking can be prevented by ensuring that the conductor bars are continually in perfect electrical contact with the rotor core. However, while such contact can perhaps be achieved in a new machine, it is impossible to guarantee that such contact is maintained throughout the service life of the machine, due for example to in-service cyclic stresses and to the different thermal expansions of the materials of the conductor bars and the rotor core.
An alternative solution to the problem of sparking is to insulate the conductor bars electrically from the rotor core. However, the selection of a satisfactory method of insulation has proved difficult. For example, wrapping the conductors in insulating tape or sheet has proved unsatisfactory because it is not possible to wrap the tape or sheet sufficiently tightly to achieve adequate tightness of the wrapped conductors in the rotor core slots to avoid fatigue failure of the bars.
Two other methods of insulation that have been proposed are disclosed in, for example, Japanese Patent No. 63-253 842 and U.S. Pat. No. 3,213,302.
Japanese Patent No. 63-253842 is concerned with the provision of insulation between a field coil and an iron core and between the turns of the field coil and proposes the use for the insulating material between the turns, of an inorganic insulating material, resin and a curing agent applied to a glass cloth. U.S. Pat. No. 3,213,302 on the other hand, discloses the use of sodium phosphate insulation with steel laminations and conductor bars as used in magnetic cores of electric power equipment.
The possibility of applying a silicon dioxide coating to copper rods in a squirrel cage motor is mentioned in U.K. Pat. No. 824 861 but such a coating is not designed to prevent sparking and would not be utilizable in the large machines with which the present invention is concerned.